Alongside these various options, there are a number of different flags that are supposed to tell the host which of these options should, and which shouldn’t, be used, prioritized, etc. The problem is, of course, that many of these flags, and many of the options, are, well, optional, which means they may or may not be implemented across different versions of code and vendor products. Hence, combining various flags with various bits of information can have a seemingly random impact on the IPv6 addresses and DNS information different hosts actually use. Perhaps the most illustrative chart is this one—

Each operating system tested seems to act somewhat differently when presented with all possible flags, and all possible sources of information. As the paper notes, this can cause major security holes. For instance, if an attacker simply brings up a DHCPv6 server on your network, and you’re not already using DHCPv6, the attacker can position itself to be a “man in the middle” for most DNS queries.

What lessons can we, as engineers and network operators, take away from this sort of thing?

First, standards bodies aren’t perfect. Standards bodies are, after all, made up of people (and a lot less people than you might imagine), and people are not perfect. Not only are people not perfect, they are often under pressures of various sorts which can lead to “less than optimal” decisions in many situations, particularly in the case of systems designed by a lot of different people, over a long stretch of time, with different pieces and parts designed to solve particular problems (corner or edge cases), and subjected to the many pressures of actually holding a day job.

Second, this means you need to be an engineer, even if you are relying on standards. In other words, don’t fall back to “but the RFC says…” as an excuse. Do the work of researching why “the RFC says,” find out what implementations do, and consider what alternatives might be. Ultimately, if you call yourself an engineer, be one.

Third, always know what is going on, on your network, and always try to account for negative possibilities, rather than just positive ones. I wonder how many times I have said, “but I didn’t deploy x, so I don’t need to think about how x interacts with my environment.” We never stop to ask if not deploying x leaves me open to security holes or failure modes I have not even considered.

Unintended consequences are, after all, unintended, and hence “Out of sight, out of mind.” But out of sight, and even out of mind, definitely does not mean out of danger.